An MRI device is a medical diagnostic imaging apparatus, which induces magnetic resonance in nuclear spins in an arbitrary section of a test subject, and provides a tomogram for the section from generated magnetic resonance signals. When a high-frequency magnetic field is irradiated on a test subject rested in a static magnetic field with a high-frequency coil (RF coil) with applying a gradient magnetic field, nuclear spins of atomic nuclei in the test subject, for example, hydrogen nuclei, are excited, and when the excited nuclear spins return to the equilibrated state, circularly polarized magnetic fields are generated as magnetic resonance signals. These signals are detected with an RF coil and subjected to signal processing to visualize hydrogen nucleus distribution in the living body.
As such an RF coil, there are transmit coil only for irradiating high-frequency magnetic fields, a receive coil only for receiving magnetic resonance signals, and a transceive coil that serves as both the coils. In order to efficiently obtain high quality images, various coils have been developed for each type of coil. For example, when nuclear spins are excited in a test subject, uniform irradiation intensity distribution is required. As for the degree of the uniformity, it is desirable that irradiation intensity in a region to be imaged is 70% or more of the maximum intensity in the irradiation intensity distribution of the region. This is because, if non-uniformity of irradiation intensity is significant, excited states of nuclear spins differ depending on sites in the test subject, and therefore non-uniformity of contrast and artifacts are induced in the obtained image. As RF coils showing such uniform irradiation intensity distribution, there are known cylindrical RF coils such as birdcage coils (refer to, for example, Patent document 1) and TEM coils (refer to, for example, Non-patent document 1).
Moreover, it is also necessary to improve irradiation efficiency. As a technique for improving the irradiation efficiency, there is the QD (Quadrature Detection) method (refer to, for example, Patent document 2, Non-patent documents 2 and 3). The QD method is a method of irradiating high-frequency magnetic fields by using two RF coils that irradiate high-frequency magnetic fields of which directions are perpendicular to each other so that phase difference of time phases of the high-frequency magnetic fields irradiated by the RF coils is 90 degrees. Since circularly polarized magnetic fields for exciting nuclear spins of hydrogen atoms can be highly efficiently irradiated by using the QD method, the irradiation intensity is theoretically improved √2 times compared with irradiation with one RF coil. Moreover, electric power is halved in terms of irradiation electric power, and therefore irradiation efficiency is improved twice. In the case of a birdcage coil or TEM coil (henceforth called cylindrical RF coil), by disposing two electric feeding ports used for irradiation at such positions that they intersect orthogonally to each other, high-frequency magnetic fields can be irradiated with one coil according to the QD method.
A cylindrical RF coil is generally used for a cylindrical (tunnel type) MRI device. Since such a tunnel type MRI device has a tunnel having a small diameter and a long length, it imposes much stress on a fat person or claustrophobic person. In order to eliminate this problem, there is desired an MRI device having a large examination space, i.e., a short tunnel of a large diameter, and thus giving superior spaciousness. Moreover, in recent years, detailed examination or treatment may be performed by disposing a contrast medium injector or a nonmagnetic treatment apparatus in the inside of an MRI device. Therefore, also in order to secure an installing space for installing various instruments near a test subject, there is desired an MRI device having a large examination space.
The tunnel type MRI device has a structure that a static magnetic field magnet, a gradient magnetic field coil, an RF shield, and an RF coil are successively disposed from the outside to the inside of the tunnel. The space inside the RF coil is the examination space in which a test subject is placed. Therefore, in order to make the examination space into which the test subject is entered larger, the internal diameter of the static magnetic field magnet located at the outermost position can be made larger. However, increase of the size of the static magnetic field magnet invites significant increase of the manufacturing cost.
There is generally required a space of 10 to 40 mm between the RF shield and the RF coil. For example, it may also be contemplated to make the examination space larger by making this distance smaller. However, if the RF shield and the RF coil are closely disposed, high-frequency eddy currents that cancel the magnetic fields to degrade the magnetic field generating efficiency are increased, and the high-frequency magnetic field distribution is suddenly changed near the RF coil to make non-uniformity of the irradiation intensity distribution of the high-frequency magnetic field in the imaging region more significant.
It is also contemplated to remove a part of coil conductors of the RF coil to enlarge the examination space. As an example of such a scheme, there is a semicylindrical birdcage coil, which corresponds to a cylindrical RF coil a part of which is removed (refer to, for example, Non-patent document 4).